Method of producing a photovoltaic device using a sputtering method

ABSTRACT

The sputtering method of the present invention comprises the steps of forming a plurality of tunnel-like magnetic fluxes on a target, forming an electric field between the target and a belt-like substrate, and conveying the belt-like substrate while reciprocating the plurality of tunnel-like magnetic fluxes at least in the direction of conveying the belt-like substrate, wherein the speed v of conveying the substrate, the distance L in the direction of conveying the belt-like substrate between two adjacent points where the magnetic field of the plurality of tunnel-like magnetic fluxes and the electric field cross each other at a right angle, and the period T of the reciprocating motion of the plurality of tunnel-like magnetic fluxes are controlled so as to L/v=(n+½)T wherein n is z−{fraction (1/16)}&lt;n&lt;z+{fraction (1/16)} and z is an integer equal to or greater than 0. The present method can solve the problem of the prior art that when a sputtering apparatus is applied to a roll-to-roll system magnetron sputtering apparatus, any part of a belt-like substrate is subjected to sputtering for a different sputtering time, thereby deteriorating the distribution of film thickness in the direction of conveying the belt-like substrate.

This is a divisional of application Ser. No. 09/076,238, filed May 12,1998. now U.S. Pat. No. 6,093,290, issued Jul. 25, 2000.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sputtering method of continuouslyforming a film on a belt-like substrate while the substrate is moved,and to a method of producing a photovoltaic device using the sputteringmethod.

2. Related Background Art

In the magnetron sputtering technique, a plasma focusing magnetgenerates a magnetic flux in the form of a tunnel, and electronsgenerated by ionization as well as secondary electrons generated bysputtering are captured by the tunnel-like magnetic flux in the vicinityof the surface of a target. As a result, there are a high density ofelectrons near the surface of the target. This brings about a highprobability of collision between the electrons and neutral gasmolecules, thereby increasing the plasma density. Therefore, themagnetron sputtering technique has advantages that the speed of forminga film by sputtering is high and that a substrate on which a film isformed is not damaged by bombardment of high-energy secondary electronsbecause the electrons are confined in the vicinity of the target surfaceby the magnetic field. However, the density of plasma increases on alocal portion of the target surface, whereby erosion of the localportion is enlarged by sputtering, whereby the target is not useduniformly. Therefore, the magnetron sputtering technique has a problemthat the target is used with a low efficiency.

In order to expand the region of the target subjected to sputtering toimprove the use efficiency of the target, the following techniques areknown. (1) The magnet assembly for generating the magnetic field ismechanically moved. (2) The location of the plasma is moved bycontrolling the distribution of effective magnetic flux density using acombination of magnets. (3) The magnet assembly is modified to improvethe distribution of magnetic flux in the region near the target, therebyexpanding the plasma region.

Japanese Patent Application Publication No. 3-51788 discloses that aplasma focusing magnet is disposed at the back side of a target, a fluxguide typically made of permalloy is disposed between the target and themagnet, and the flux guide is rocked to move the concentrated region ofthe magnetic flux.

FIG. 8 is a cross-sectional view for showing an example of a magnetronsputtering apparatus including a magnet assembly adapted to bemechanically moved. In FIG. 8, a target 1 is disposed on a targetcooling plate 2 such that they are in intimate contact with each other.At the back side of the cooling plate 2 (i. e., the side opposite to theside of locating the target), there is disposed a magnet assembly 6including permanent magnets 3 and 4 and a permanent magnet supportingmember 5. The adjacent magnets 3 and 4 disposed on the side of thecooling plate 2 have magnetic poles opposite to each other such thatlines of magnetic force emerging from the magnet assembly 6 returns tothe magnet assembly 6 after passing through the surface region of thetarget 1, that is, a closed loop shape of tunnel-like magnetic flux isformed as shown by broken lines in FIG. 8. To increase the utilizationefficiency of the target 1, the tunnel-like magnetic flux is rocked bymoving the magnet assembly 6 along a circular path (means for moving themagnet is not shown in FIG. 8). A negative DC voltage or a highfrequency voltage is applied to the target 1 (from a power supply notshown in FIG. 8) to generate a plasma, thereby sputtering the target.

As the method of successively forming functional deposited films of aphotovoltaic device on a belt-like substrate, there is proposed a methodof arranging a plurality of deposition chambers independent from oneanother and forming respective semiconductor layers in the respectivedeposition chambers. U.S. Pat. No. 4,400,409 discloses a continuousplasma CVD apparatus employing a roll-to-roll system. In this apparatus,a plurality of glow discharging regions are arranged. A flexiblesubstrate with a sufficiently large length and a desired width iscontinuously moved in a longitudinal direction such that it is passedthrough the plurality of glow discharging regions from one region toanother and depositing respective semiconductor layers of desiredelectroconductive types when it passes through the respective glowdischarging regions. This technique makes it possible to continuouslyproduce devices having semiconductor junctions. In the patent citedabove, dopant gases used in each step of the semiconductor layerproduction are isolated from one another by gas gates so as to preventthe dopant gases from diffusing from one region into another thuspreventing the glow discharging regions from being contaminated. Morespecifically, the glow discharging regions are isolated from each otherby slit-like isolation paths through which isolation gas such as Ar orH₂ is passed.

However, when such a roll-to-roll system is implemented with magnetronsputtering apparatus of the type described above, the speed of thetunnel-like magnetic flux varies as a sine wave in the direction ofmoving the belt-like substrate because the magnet assemblies are movedalong circular paths while the belt-like substrate is moved at aconstant speed. In other words, the speed of the tunnel-like magneticflux has alternately positive and negative values. As a result, thesputtering time varies depending on the location of the plate-likesubstrate. This bring about nonuniformity of the film thickness alongthe direction of moving belt-like substrate.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided asputtering method comprising the steps of forming a plurality oftunnel-like magnetic fluxes on a target, forming an electric fieldbetween the target and a belt-like substrate, and conveying thebelt-like substrate while reciprocating the plurality of tunnel-likemagnetic fluxes at least in the direction of conveying the belt-likesubstrate, wherein the speed v of conveying the substrate, the distanceL in the direction of conveying the belt-like substrate between twoadjacent points where the magnetic field of the plurality of tunnel-likemagnetic fluxes and the electric field cross each other at a rightangle, and the period T of the reciprocating motion of the plurality oftunnel-like magnetic fluxes are controlled so as to satisfy L/v=(n+½)Twherein n is z−{fraction (1/16)}<n<z+{fraction (1/16)} and z is aninteger equal to or greater than 0.

According to another aspect of the invention, there is provided asputtering method comprising the steps of forming a plurality oftunnel-like magnetic fluxes of a closed loop shape on a target, formingan electric field between the target and a belt-like substrate, andconveying the belt-like substrate while reciprocating the plurality oftunnel-like magnetic fluxes at least in the direction of conveying thebelt-like substrate, wherein the speed v of conveying the substrate, theinterval p of disposing the plurality of closed loops, and the period Tof the reciprocating motion of the plurality of tunnel-like magneticfluxes are controlled so as to satisfy p/v=(n+1/m₁)T wherein n is z−1/(8m₁)<n<z+1/(8 m₁), z is an integer equal to or greater than 0, and m₁ isa number of closed loops mutually canceling nonuniformity.

According to still another aspect of the invention, there is provided asputtering method comprising the steps of forming a tunnel-like magneticflux on a target, forming an electric field between the target and abelt-like substrate, and conveying the belt-like substrate whilereciprocating the tunnel-like magnetic flux at least in the direction ofconveying the belt-like substrate, wherein the target is present inplurality and the plurality of targets reciprocate independent of oneanother, and wherein the speed v of conveying the substrate, thedistance d between two adjacent centers of the plurality ofindependently reciprocating targets, and the period T of thereciprocating motion of the tunnel-like magnetic flux are controlled soas to satisfy d/v=(n+1/m₂)T wherein n is z−1/(8 m₂)<n<z+1/(8 m₂), z isan integer equal to or greater than 0, and m₂ is a number of targetsmutually canceling nonuniformity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematically cross-sectional view for showing a cathodeunit according to the present invention, taken along the line 1—1 ofFIG. 2;

FIG. 2 is a schematically perspective view for showing a closed loop ofa tunnel-like magnetic flux formed on a target;

FIG. 3 is a graph illustrating the speed of a tunnel-like flux in thedirection of conveying a belt-like substrate;

FIG. 4 is a schematically cross-sectional view for showing aroll-to-roll system sputtering apparatus according to the presentinvention;

FIG. 5 is a schematic view for showing a mask for use in producing atransparent and electroconductive ITO film;

FIG. 6 is a schematically cross-sectional view for showing aphotovoltaic device comprising a back reflector formed by means of thesputtering method according to the present invention;

FIG. 7 is a schematically cross-sectional view for showing anotherroll-to-roll system sputtering apparatus according to the presentinvention;

FIG. 8 is a schematically cross-sectional view for showing a cathodeunit;

FIG. 9 is a schematically cross-sectional view for showing a cathodeunit according to the invention;

FIG. 10 is a schematically perspective view for showing closed loops oftunnel-like magnetic fluxes formed on a target;

FIG. 11 is a schematically cross-sectional view for showing anotherroll-to-roll system sputtering apparatus according to the presentinvention; and

FIG. 12 is a graph for showing the relationship between the variation ofa short-circuit current and n.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 2 is a perspective view schematically illustrating an example of atarget according to the present invention. FIG. 1 is a cross-sectionalview for showing an example of a cathode unit and showing a crosssection of the target, taken along the line 1—1 of FIG. 2. A target 1 isdisposed on a target cooling plate 2 such that they are in intimatecontact with each other and such that the target 1 is properly spacedfrom a belt-like substrate 10. At the back side of the cooling plate 2,there is disposed a magnet assembly 6 including permanent magnets 3 and4 and a permanent magnet supporting member 5. In this embodiment, themagnets 3 are disposed on the periphery of the supporting member 5 suchthat the same magnetic poles are directed to the upper side and themagnet 4 is disposed at the center of the supporting member 5 so thatits magnetic pole is opposite in polarity to that of the magnet 3. Themagnets may be either of the permanent type or of the electromagnettype. The magnets may be disposed on the supporting member 5 without anyspace between them, if desired.

Because the magnetic poles of the magnets 3 and 4 on the side of thecooling plate 2 are opposite to each other, lines of magnetic forceemerging from the magnet assembly 6 returns to the magnet assembly 6after passing through the surface region of the target 1, whereby aclosed loop of tunnel-like magnetic flux 7 is formed. To improve theutilization efficiency of the target 1, the magnet assembly 6 is movedalong a circular path so that the tunnel-like magnetic flux is moved onthe target (means for moving the magnet assembly is not shown in thedrawings). A negative DC voltage or a high frequency voltage is appliedto the target 1 (from a power supply not shown in the drawings) so thatan electric field is formed in a direction perpendicular to thesubstrate and also to the target. In the above-described manner, aplasma is generated and the target is sputtered.

The shape of the magnetic flux on the surface of the target is shown inFIG. 2. As shown, a magnetic tunnel is formed with a magnetic flux 7 ofa closed loop shape.

The belt-like substrate 10 is moved at a speed v. The tunnel-likemagnetic flux is moved along a circular path with a period of T by themotion of the magnet assembly 6. The distance in the motion direction ofthe belt-like substrate between portions where the magnetic field of thetunnel-like magnetic flux and the electric field generated by theelectric power applied to an electrode cross at a right angle is set toL.

Because the magnet assembly 6 is moved along the circular path, thespeed of moving the tunnel-like magnetic flux in the direction of movingthe belt-like substrate varies as a sine wave shown in FIG. 3. In otherwords, the speed of the tunnel-like magnetic flux has alternatelypositive and negative values (herein the speed is defined as beingpositive in the same direction as the motion of the substrate). That is,the tunnel-like magnetic flux reciprocates in the longitudinal directionof the belt-like substrate.

In this invention, the reciprocating motion of the tunnel-like magneticflux is controlled such that when the belt-like substrate moves adistance in the direction of moving the belt-like substrate between twoadjacent points where which the magnetic field of the tunnel-likemagnetic flux and the electric field formed between the target and thesubstrate cross each other at a right angle, the reciprocating motion ofthe tunnel-like magnetic flux becomes opposite in direction. Morespecifically in FIG. 1, the speed of moving the tunnel-like magneticflux in the direction of the motion of the belt-like substrate when thebelt-like substrate passes through the region A is substantially equalbut opposite in direction to that when the belt-like substrate passesthrough the region B (at a position away from the region A by thedistance L between two adjacent points where the magnetic field of thetunnel-like magnetic flux and the electric field between the target andthe substrata cross each other at a right angle).

That is, the portion of the substrate which is located in the region Aat a certain time will be located, after L/v hour, in the region B apartfrom the region A by a distance L. The reciprocating motion of thetunnel-like magnetic flux (FIG. 3) is preferably controlled so as tohave about ½ of the phase in the region A. This requirement can berepresented by an equation such that L/v=(n+½)T wherein n is an integerequal to or greater than 0. When the above requirement is satisfied, therelative speed of the belt-like substrate with respect to thetunnel-like magnetic fluxes in the direction of the motion of thebelt-like substrate can be regarded as a constant value for any part ofthe belt-like substrate over the whole target. In other words, any partof the belt-like substrate is subjected to sputtering for substantiallythe same sputtering time. In the above equation representing thecondition which should be met to form an uniform film, n is allowed tohave some deviation such as z−{fraction (1/16)}<n<z+{fraction (1/16)}(wherein z is an integer equal to or greater than zero). In other words,a deviation within ±{fraction (1/16)} of the period from the half-periodis allowed.

In the present embodiment, the width of the substrate is sufficientlysmall compared with that of the magnet assembly, and thus no significantvariation occurs in the width direction.

Second Embodiment

FIG. 9 is a cross-sectional schematically illustrating another exampleof a cathode unit in which two tunnel-like magnetic fluxes are formed.In this cathode unit, as shown in FIG. 10, two closed loops oftunnel-like magnetic fluxes are formed on the surface of the target 1.This structure brings about more uniform erosion of the target and thusa higher utilization efficiency.

More specifically, the speed of the tunnel-like magnetic flux in thedirection of the motion of the belt-like substrate when the belt-likesubstrate passes through the region A shown in FIG. 9 (wherein themagnetic field of the tunnel-like magnetic flux and the electric fieldbetween the target and the substrata cross each other at a right angle)is equal but opposite in direction to that when the belt-like substratepasses through the region C in FIG. 9 (wherein the magnetic field of thetunnel-like magnetic flux and the electric field between the target andthe substrata cross each other at a right angle). That is, when thedistance in the direction of the motion of the belt-like substratebetween the two tunnel-like magnetic fluxes each of a closed loop shapeis denoted by p, the period T of the reciprocating motion of the twotunnel-like magnetic fluxes on the target is controlled so as to satisfyp/v=(n+½)T (wherein n is an integer equal to or greater than zero) togenerate a phase difference of T/2.

Because the two tunnel-like magnetic fluxes each of a closed loop shapeare similar in shape and apart from each other by distance p, the speedof the tunnel-like magnetic flux in the direction of the motion of thebelt-like substrate when the belt-like substrate passes through theregion B (wherein the magnetic field of the tunnel-like magnetic fluxand the electric field between the target and the substrate cross eachother at a right angle) is also equal but opposite in direction to thatwhen the belt-like substrate passes through the region D (wherein themagnetic field of the tunnel-like magnetic flux and the electric fieldbetween the target and the substrata cross each other at a right angle).Thus, the relative speed of the belt-like substrate with respect to thetunnel-like magnetic fluxes in the direction of the motion of thebelt-like substrate can be regarded as a constant value for any part ofthe belt-like substrate over the whole target. In other words, any partof the belt-like substrate is subjected to sputtering for substantiallythe same sputtering time.

Alternatively, the speed of the two tunnel-like magnetic fluxes of aclosed loop shape may also be controlled such that the speed becomesequal but opposite in direction between the region A and the region Band also between the region C and the region D as is the case in theprevious embodiment of using only one tunnel-like magnetic flux of aclosed loop shape.

Although the present embodiment employs two tunnel-like magnetic fluxeseach of a closed loop shape in each vacuum chamber, one cathode unit mayalso include three tunnel-like magnetic fluxes each of a closed loopshape. In this case, the rocking motion is controlled so as to generatea phase difference of T/3 between adjacent loops and thus thicknessnonuniformity produced at the first tunnel-like magnetic flux iscompensated by the second and third tunnel-like magnetic fluxes, therebyensuring that the film thickness becomes uniform in the longitudinaldirection of the belt-like substrate.

Furthermore, one cathode unit may include four tunnel-like magneticfluxes each of a closed loop shape and the rocking motion may becontrolled so as to generate a phase difference of T/4 between adjacentloops. In this case, nonuniformity of thickness produced at the firsttunnel-like magnetic flux is compensated by the third tunnel-likemagnetic flux, and nonuniformity of thickness produced at the secondtunnel-like magnetic flux is compensated by the fourth tunnel-likemagnetic flux, thereby ensuring that the film has improved uniformity ofthickness in the longitudinal direction of the belt-like substrate. Whenthe rocking motion is controlled so as to generate a phase difference ofT/2 between adjacent loops, then nonuniformity of thickness produced atthe first tunnel-like magnetic flux is compensated by the secondtunnel-like magnetic flux, and nonuniformity of thickness produced atthe third tunnel-like magnetic flux is compensated by the fourthtunnel-like magnetic flux.

More generally, one cathode unit may include a plurality of tunnel-likemagnetic fluxes each of a closed loop shape. In this general case, therocking motion is controlled such that p/v=(n+1/m₁)T wherein n is aninteger equal to or greater than zero, v is the conveying speed of thebelt-like substrate, m₁ is a number of tunnel-like magnetic fluxes eachof a closed loop shape in the same cathode unit for cancelingnonuniformity of thickness, p is the pitch of the tunnel-like magneticfluxes each of a closed loop shape arranged in the direction of themotion of the belt-like substrate, and T is the period of the rockingmotion of the tunnel-like magnetic fluxes each of a closed loop shape.In the above equation representing the condition required to achievegood uniformity of film thickness, a value of n may not always is aninteger, but it must be within the range of z−1/(8 m₁) <n<z+1/(8 m₁)(wherein z is an integer equal to or greater than zero).

FIG. 4 is a schematic view for illustrating an example of a sputteringapparatus according to the present invention. In this apparatus, asubstrate feeding vacuum chamber 11, a film formation vacuum chamber 12,and a substrate wind-up vacuum chamber 13 are connected from chamber tochamber via a gas gate 14, and evacuated via an exhaust vent 15 by avacuum pump (not shown in the drawing). A belt-like substrate 10 woundaround a reel 16 is fed into the film formation vacuum chamber 12 alongthe direction as shown by the arrow in FIG. 4 after the conveyingdirection is changed by a transfer roller 17. In the film formationvacuum chamber 12, the belt-like substrate 10 is heated by a lamp heater18 to a predetermined film formation temperature. In film formationregions 19 a and 19 b, films are formed on the belt-like substrate 10 bycathode units 9 a and 9 b provided with various targets. Each cathodeunit 9 a and 9 b has a structure such as that shown in FIG. 1 or 9 forforming similar tunnel-like magnetic fluxes on the surfaces of therespective targets and can be rocked as described above. Aftercompletion of the film formation, the belt-like substrate 10 is conveyedinto the substrate wind-up vacuum chamber 13 and wound around a wind-upreel 20 wherein the conveying direction of the belt-like substrate 10 ischanged by a conveying roller 17 in the middle of the conveying path. Apurge gas is supplied through a purge gas supply line 21 and is passedthrough the gas gates 14 so as to prevent the vacuum chambers from beingcontaminated with a gas from other vacuum chambers.

Although in the above embodiments the magnet assembly for generating themagnetic field is mechanically moved along a circular path, the presentinvention is not limited thereto. The magnet assembly may be moved inany manner as long as the motion of the magnet assembly causes thetunnel-like magnetic flux of a closed loop shape to periodically move onthe target in the direction of motion of the substrate. For example, themagnet assembly may be moved in a simply reciprocating manner in theconveying direction of the substrate.

Third Embodiment

FIG. 11 is a cross-sectional view illustrating another embodiment of asputtering apparatus according to the present invention. Film formationregions 19 a and 19 b are provided in a vacuum chamber 12 a. Similarly,film formation regions 19 c and 19 d are provided in a vacuum chamber 12b. Cathode units 9 a and 9 b are disposed in the film formation regions19 a and 19 b, respectively, and cathode units 9 c and 9 d are disposedin the film formation regions 19 c and 19 d, respectively. The same kindof film is formed in the film formation regions 19 a and 19 b which workin cooperation with each other. Similarly, another film is deposited inthe film formation regions 19 c and 19 d which work in cooperation witheach other. The cathode units are disposed such that each of a distancebetween the centers of the cathode units 9 a and 9 b and a distancebetween the centers of the cathode units 9 c and 9 d is d. Each cathodeunit 9 a, 9 b, 9 c, and 9 d has a structure such as that shown in FIG. 1or 9, and can be moved independently along a circular path.

Because the magnet assemblies 5 are moved along circular paths, thespeed of the tunnel-like magnetic fluxes varies as a sine wave in thedirection of moving the belt-like substrate, as shown in FIG. 3, and thespeed of the tunnel-like magnetic flux has alternately positive andnegative values. That is, the tunnel-like magnetic fluxes reciprocatesin the longitudinal direction of the belt-like substrate. Thereciprocating motion is preferably controlled such that when a certainpoint of the belt-like substrate comes from the cathode unit 9 a to thecathode unit 9 b, the reciprocating motion of the tunnel-like magneticflux formed by the cathode unit becomes opposite in the direction.

More specifically, the motion of the cathode units 9 a and 9 b iscontrolled by a speed controller 30 a such that d/v=(n+½)T, that is,there is a phase difference equal to half the rocking period T betweenthe cathode units 9 a and 9 b, in other words, such that when thebelt-like substrate 10 passes over the first cathode unit 9 a, aperiodic variation in film thickness in the longitudinal direction ofthe belt-like substrate 10 is produced, this variation in film thicknessis compensated by a thickness variation produced when the belt-likesubstrate 10 passes over the second cathode unit 9 b. Similarly, themotion of the cathode units 9 c and 9 d is controlled by a speedcontroller 30 b. The other parts of the apparatus are similar to thoseof the apparatus shown in FIG. 4.

Although in the present embodiment, two cathode units are disposed ineach vacuum chamber, three cathode units may be disposed in each vacuumchamber. In this case, the motion of cathode units is controlled suchthat there is a phase difference equal to ⅓ of the rocking period amongthe cathode units and thus nonuniformity of thickness produced at thefirst cathode unit is compensated by the second and third cathode units,thereby ensuring that the film has improved uniformity of thickness inthe longitudinal direction of the belt-like substrate.

Furthermore, four cathode units may be disposed in a single vacuumchamber and the rocking motion may be controlled such that there is aphase difference of T/4 between adjacent cathode units. In this case,nonuniformity of thickness produced at the first cathode unit iscompensated by the third cathode unit, and nonuniformity of thicknessproduced at the second cathode unit is compensated by the fourth cathodeunit. When the rocking motion is controlled such that there is a phasedifference of T/2 between adjacent cathod units, then nonuniformity ofthickness produced at the first cathode unit is compensated by thesecond cathode unit, and nonuniformity of thickness produced at thethird cathode unit is compensated by the fourth cathode unit.

More generally, a plurality of cathode units may be disposed in a singlevacuum chamber. In this general case, the rocking motion is controlledsuch that d/v=(n+1/m₂)T wherein n is an integer equal to or greater thanzero, v is the conveying speed of the belt-like substrate, m₂ is thenumber of cathode units among which nonuniformity of thickness iscanceled, d is the distance between adjacent centers of cathode units inthe direction of the motion of the belt-like substrate, and T is theperiod of the rocking motion of the cathode units. In the above equationrepresenting the condition required to achieve good uniformity of filmthickness, provided that n may not always be an integer, but n must bewithin the range of z−1/(8 m₂)<n<z+1/(8 m₂) (wherein z is an integerequal to or greater than zero).

The present invention is described in further detail below withreference to specific Examples. It should be understood that theseExamples are intended to be illustrative only and that the invention isnot limited to those Examples.

EXAMPLE 1

By using the apparatus shown in FIG. 4 including cathode units 9 a and 9b of the type similar to the cathode unit 9 shown in FIG. 1, a backreflector including a thin Ag film and a thin ZnO film serving as alower electrode and a light reflecting layer was formed using thetunnel-like magnetic flux of a closed loop as shown in FIG. 2.Semiconductor layers were further deposited thereon, and then aphotovoltaic device were produced.

As the belt-like substrate 10, SUS430BA sheet (with a width of 120 mm, alength of 100 m, and a thickness of 0.13 mm) was employed. The belt-likesubstrate 10 was well degreased and cleaned. The cleaned belt-likesubstrate 10 was stretched between the two reels as shown in FIG. 4 andthe tension was adjusted so that there is no slack. The apparatus wasevacuated from exhaust vents 15 of the vacuum-tight chambers 11, 12, and13 by a vacuum pump (not shown). The lamp heater 18 was then turned onso that the belt-like substrate 10 was heated to a film formationtemperature of 400° C. Ar gas serving as a purging gas was introducedvia the purge gas supply inlets 21. Ar gas serving as a sputtering gaswas then supplied at a flow rate of 50 sccm into the respective filmformation regions 19 a and 19 b via a source gas supply line (notshown). The conductance was reduced by closing a main valve (not shown)so that the pressure is maintained at a film formation pressure of2.0×10⁻³ Torr.

While moving the substrate, a negative DC voltage was applied from anexternal DC power supply (not shown) to the cathode unit 9 a on which aAg target was loaded and also to the cathode unit 9 b on which a ZnOtarget was loaded to generate a discharge and successively depositing,by means of sputtering, a thin Ag film and a thin ZnO film on thesubstrate. After completion of the film deposition, the substrate waswound around the wide-up reel 12. Thus, the substrate with a backreflector was obtained.

In the above process, the belt-like substrate was moved at a conveyingspeed v of 200 mm/min, the magnet assembly was moved along the circularpath with a period of 1 min, and the distance L in the direction of themotion of the belt-like substrate between portions where the magneticfield of the tunnel-like flux and the electric field generated byelectric power applied between electrodes cross at a right angle was setto 100 mm. The conveying speed of the belt-like substrate was detected,and the resultant value was fed back to an automatic control system sothat the rocking period T of the magnet satisfied the conditionL/v=(n+{fraction (1/2)})T (wherein n=0 in this Example). The conveyingspeed of the substrate was maintained at the fixed value with afluctuation of less than 1%.

The belt-like substrate on which the back reflector was deposited in theabove-described manner using the above apparatus was taken out of theroll-to-roll system apparatus, and cut into a plurality of sheets with asize of 5 cm×5 cm. A sheet of substrate was placed in a CVD apparatuswith a single vacuum chamber, and an n-type non-single-crystal siliconsemiconductor film, an i-type non-single-crystal silicon semiconductorfilm, and a p-type non-single-crystal silicon semiconductor film weresuccessively deposited on the sheet of the substrate by means of CVDunder the conditions shown in Table 1, thereby obtaining a multilayerstructure of semiconductor layers, and the semiconductor device wascompleted.

TABLE 1 Substrate Film Gas Flow Pres- Discharge Temperature (Thickness,Rate sure Power (set temp.) nm) (sccm) (Torr) (W) (° C.) n-type SiH₄:150 1.0 RF 350 non-single- PH₃: 3 (150) crystal H₂: 1500 silicon (20)i-type SiH₄: 60 0.01 Microwave 300 non-single- H₂: 200 (200) crystalsilicon (400) p-type SiH₄: 5 1.0 RF 250 non-single- 1% BF₃: 1 (700)crystal H₂: 2000 silicon (20)

The obtained substrate was put together with a stainless steel mask 901having 25 openings 902 with a diameter of 6 mm as shown in FIG. 5, intoa vacuum evaporation apparatus with a single vacuum chamber, and atransparent and electroconductive ITO (indium tin oxide) film wasdeposited on the semiconductor film through the openings 902 by means ofvacuum evaporation under the conditions shown in Table 2, therebyproducing a photovoltaic device having the structure whose cross sectionis schematically shown in FIG. 6.

TABLE 2 Evaporation Source In—Sn Alloy (Composition Ratio) (50:50)Evaporation Ambient O₂: 3 × 10⁻⁴ (Torr) Substrate Temperature 180 (° C.)Evaporation Rate 0.1 (nm/sec) Filrn Thickness 70 (nm)

As shown in FIG. 6, the obtained device had the belt-like substrate 10;the back reflector film 911 consisting of the thin Ag film 912 and thethin ZnO film 913; the semiconductor multilayer 921; and the transparentand electroconductive ITO film 931, wherein the semiconductor multilayer921 consists of the n-type non-single crystal silicon semiconductor film922, the i-type non-single-crystal silicon semiconductor film 923, andthe p-type non-single-crystal silicon semiconductor film 924.

The obtained photovoltaic devices were evaluated. Very little variationin the short-circuit current (Jsc) in the conveying direction of thebelt-like substrate was observed. As to only the back reflector film,interference color due to the nonuniform thickness of the ZnO film wasnot observed.

A similar experiment was performed but the rocking period T of themagnet assembly was shifted to various values from the ideal valuesatisfying L/v=(n+½)T (wherein n=0 in this experiment). The result isshown in FIG. 12. As can be seen from FIG. 12, very little variation inthe short-circuit current and interference colors were observed when nwas within the range of n=0±{fraction (1/16)}. However, when n was outof the above range, large variation up to 4% in the short-circuitcurrent were observed and periodic interference patterns caused bynonuniformity of film thickness were observed.

EXAMPLE 2

Photovoltaic devices comprising a thin ZnO film and a semiconductormultilayer structure were produced in the same manner as in Example 1described above except that instead of the thin Ag film a thin Al filmwas deposited using an Al target as the target 1. In this Example, Alwas deposited at room temperature, and ZnO was deposited at 150° C. (settemperature).

The obtained photovoltaic devices were evaluated. Very little variationin the short-circuit current (Jsc) in the conveying direction of thebelt-like substrate was observed. As to only the back reflector film,interference color due to the nonuniform thickness of the ZnO film wasnot observed.

A similar experiment was performed but the rocking period T of themagnet assembly was shifted to various values from the ideal valuesatisfying L/v=(n+½)T (wherein n=0 in this experiment). The result wasvery similar to that of Example 1. That is, very little variation in theshort-circuit current and interference colors were observed when n waswithin the range of n=0±{fraction (1/16)}. However, when n was out ofthe above range, large variation up to 3% in the short-circuit currentwere observed and periodic interference patterns caused by nonuniformityof film thickness were observed.

EXAMPLE 3

A thin Ag film and a thin ZnO film were deposited in the same manner asin Example 1, and a plurality of semiconductor films were successivelydeposited thereon using the apparatus shown in FIG. 7, thereby producinga photovoltaic device.

A back reflector film was first formed using the apparatus shown in FIG.4 under the same conditions as those employed in Example 1. Thebelt-like substrate on which the back reflector film was formed wastaken out of the apparatus shown in FIG. 4 and put into a roll-to-rollsystem CVD apparatus shown in FIG. 7.

The CVD apparatus shown in FIG. 7 includes a substrate feeding vacuumchamber 71, an n-type semiconductor layer deposition vacuum chamber 72,an i-type semiconductor layer deposition vacuum chamber 73, a p-typesemiconductor layer deposition vacuum chamber 74, and a substratewind-up vacuum chamber 75 which are connected from chamber to chambervia a gas gate 76. The CVD apparatus was evacuated through exhaust vents77 by a vacuum pump (not shown). The belt-like substrate 70 on which theback reflector film was formed was wound around a feeding reel 78. Theconveying direction of the belt-like substrate 70 wound around thefeeding reel was changed by a conveying roller 79, and the substrate wasfed into the n-type semiconductor layer formation vacuum chamber 72,then the i-type semiconductor layer formation vacuum chamber 73 andfinally the p-type semiconductor layer formation vacuum chamber 74 (asshown by the arrow of FIG. 7). After completion of forming the films onthe belt-like substrate 70 in the respective vacuum chambers, thebelt-like substrate 70 was changed in the moving direction by aconveying roller 79 and wound around a wind-up reel 80. A purge gas wassupplied through a purge gas supply line 81 and was passed through thegas gates 76, thereby preventing the vacuum chamber from beingcontaminated with gas from other vacuum chambers.

While evacuating the vacuum chambers 71 to 75 by a vacuum pump (notshown) via the exhaust vents 77, the substrate was heated in arespective vacuum chamber for forming a film to a predeterminedtemperature by lamp heaters 82. Furthermore, a film formation gas wasintroduced through a film formation gas inlet 83, and H₂ serving as thepurge gas was introduced through the purge gas supply line 81. RF powerwith a frequency of 13.56 MHZ was applied to a discharging electrode 84,and microwave power with a frequency of 2.45 GHz was applied into thechamber 73 through microwave waveguide means 85, thereby generating aglow discharge in each chamber and forming films on the belt-likesubstrate 70 by means of CVD under the conditions shown in Table 3.

TABLE 3 Sub- strate Tempe- Film Film Dis- rature forma- (Thick- Gas FlowPre- charge (set tion ness, Rate ssure Power temp.) chamber nm) (sccm)(Torr) (W) (° C.) 72 n-type SiH₄: 150 1 RF 350 non- PH₃: 3 (150) single-H₂: 1500 crytstl silicon (20) 73 i-type SiH₄: 60 0.01 Micro- 300 non-H₂: 260 wave single- (200) crystal silicon (400) 74 p-type SiH₄: 5 1 RF250 non- 1% BF₃: 1 (700) single H₂: 2000 crystal (20)

The belt-like substrate 70 on which the non-single crystal silicon filmswere deposited in the above-described manner using the above apparatuswas taken out of the roll-to-roll apparatus, and cut into a plurality ofsheets with a size of 5 cm×5 cm. Furthermore, a transparent andelectroconductive ITO film was formed thereon under the same conditionsas those employed in Example 1 to produce photovoltaic devices.

The obtained photovoltaic devices were evaluated. Very little variationin the short-circuit current (Jsc) in the conveying direction of thebelt-like substrate was observed. As to only the back reflector film,interference color due to the nonuniform thickness of the ZnO film wasnot observed.

A similar experiment was performed but the rocking period T of themagnet assembly was shifted to various values from the ideal valuesatisfying L/v=(n+½)T (wherein n=0 in this experiment). Very littlevariation in the short-circuit current and interference colors wereobserved when n was within the range of n=0±{fraction (1/16)}. However,when n was out of the above range, large variation up to 4% in theshort-circuit current were observed, and periodic interference patternscaused by nonuniformity of film thickness were observed.

EXAMPLE 4

By using the apparatus shown in FIG. 4 including a cathode unit of thetype similar to the cathode unit 9 shown in FIG. 9, a thin Ag film and athin ZnO film were formed in the same manner as in Example 1 except thattwo closed loops of tunnel-like magnetic fluxes such as those shown inFIG. 10 were used. Semiconductor layers were further formed thereon andphotovoltaic devices were produced. In the above process, the belt-likesubstrate was moved at a conveying speed v of 200 mm/min, the magnetassembly was moved along the circular path with a period of 30 sec, andthe pitch p in the direction of the motion of the belt-like substratebetween two tunnel-like fluxes was set to 50 mm. The conveying speed ofthe belt-like substrate was detected, and the resultant value was fedback to an automatic control system so that the rocking period T of themagnet satisfied the condition p/v=(n+1/m₁) T (wherein m₁=2 and n=0).

A similar experiment was also performed but the rocking period T of themagnet assembly was shifted to various values from the ideal valuesatisfying p/v=(n+1/m₁) T (wherein m₁=2 and n=0). Very little variationin the short-circuit current and interference colors were observed whenn was within the range of n=0±{fraction (1/16)}. However, when n was outof the above range, large variation up to 4% in the short-circuitcurrent were observed and periodic interference patterns caused bynonuniformity of film thickness were observed.

EXAMPLE 5

Photovoltaic devices including a thin ZnO film and a semiconductormultilayer structure were produced in the same manner as in Example 4described above except that instead of the thin Ag film a thin Al filmwas deposited using an Al target as the target 1. In this example, Alwas deposited at room temperature, and ZnO was deposited at 150° C.

The obtained photovoltaic devices were evaluated. Very little variationin the short-circuit current (Jsc) in the conveying direction of thebelt-like substrate was observed. As to only the back reflector film,interference color due to the nonuniform thickness of the ZnO film wasnot observed.

A similar experiment was performed but the rocking period T of themagnet assembly was shifted to various values from the ideal valuesatisfying p/v=(n+1/m₁)T (wherein m₁=2 and n=0). Very little variationin the short-circuit current and interference colors were observed whenn was within the range of n=0±{fraction (1/16)}. However, when n was outof the above range, large variation up to 3% in the short-circuitcurrent were observed and periodic interference patterns caused bynonuniformity of film thickness were observed.

EXAMPLE 6

A thin Ag film and a thin ZnO film were deposited in the same manner asin Example 4, and a plurality of semiconductor films were successivelydeposited thereon using the apparatus shown in FIG. 7, thereby producingphotovoltaic devices.

The obtained photovoltaic devices were evaluated. Very little variationin the short-circuit current (Jsc) in the conveying direction of thebelt-like substrate was observed. As to only the back reflector film,interference color due to the nonuniform thickness of the ZnO film wasnot observed.

A similar experiment was also performed but the rocking period T of themagnet assembly was shifted to various values from the ideal valuesatisfying p/v =(n+1/m₁)T (wherein m₁=2 and n=0). Very little variationin the short-circuit current and interference colors were observed whenn was within the range of n=0±{fraction (1/16)}. However, when n was outof the above range, large variation up to 3% in the short-circuitcurrent were observed and periodic interference patterns caused bynonuniformity of film thickness were observed.

EXAMPLE 7

Using the apparatus shown in FIG. 11, a back reflector including a thinAg film and a thin ZnO film serving as a lower electrode and a lightreflecting layer was formed. Semiconductor layers were further formedthereon, thereby producing photovoltaic devices.

As the belt-like substrate 10, SUS430BA sheet (with a width of 120 mm, alength of 100 m, and a thickness of 0.13 mm) was employed. The belt-likesubstrate 10 was well degreased and cleaned. The cleaned belt-likesubstrate 10 was stretched between the two reels as shown in FIG. 11 andthe tension was adjusted such that there is no slack. The apparatus wasevacuated from exhaust vents 15 of the vacuum chambers 11, 12 a, 12 b,and 13 by using a vacuum pump (not shown). The belt-like substrate 10was heated to a formation temperature of 400° C. using lamp heaters 18.Ar gas serving as a purging gas was introduced via the purge gas supplyinlets 21. Ar gas serving as a sputtering gas was then supplied at aflow rate of 50 sccm into the respective film formation regions 19 a, 19b, 19 c, and 19 d via a source gas line (not shown). The conductance isreduced by closing a main valve (not shown) so that the pressure ismaintained at a film formation pressure of 2.0×10⁻³ Torr. A negative DCvoltage was applied from an external DC power supply (not shown) to thecathode units 9 a and 9 b on which a Ag target was loaded and also tothe cathode units 9 c and 9 d on which a ZnO target was loaded so as togenerate a discharge, thereby successively depositing, by means ofsputtering, a thin Ag film and a thin ZnO film on the substrate. Aftercompletion of the film deposition, the substrate was wound around thewind-up reel 20. Thus, a back reflector film was obtained.

In the above process, the belt-like substrate was moved at a conveyingspeed v of 400 mm/min, the two cathode units 9 a and 9 b (9 c and 9 d)on which the same kind of target was loaded were disposed such that thepitch d between them becomes 450 mm, and the magnet assemblies weremoved with a period of 15 sec, The motion of the assemblies wascontrolled by the controller 30 a and 30 b such that the conditionp/v=(n+1/m₂)T (wherein m₂=2 and n=4) was satisfied.

The belt-like substrate on which the back reflector was deposited in theabove-described manner using the above apparatus was taken out of theroll-to-roll apparatus, and photovoltaic devices having the structureschematically shown in the cross-sectional view of FIG. 6 were producedin the same manner as in Example 1.

The obtained photovoltaic devices were evaluated. Very little variationin the short-circuit current (Jsc) in the conveying direction of thebelt-like substrate was observed. As to only the back reflector film,interference color due to the nonuniform thickness of the ZnO film wasnot observed.

A similar experiment was performed but the rocking period T of themagnet assemblies was shifted to various values from the ideal valuesatisfying p/v=(n+1/m₂) T (wherein m₂=2 and n=4). Very little variationin the short-circuit current and interference colors were observed whenn was within the range of n=4±{fraction (1/16)}. However, when n was outof the above range, large variation up to 4% in the short-circuitcurrent were observed and periodic interference patterns caused bynonuniformity of film thickness were observed.

EXAMPLE 8

Photovoltaic devices including a thin ZnO film and a semiconductormultilayer structure were produced in the same manner as in Example 7described above except that instead of the thin Ag film a thin Al filmwas deposited using an Al target as the target 1. In this Example, Alwas deposited at room temperature, and ZnO was deposited at 150° C.

The obtained photovoltaic devices were evaluated. Very little variationin the short-circuit current (Jsc) in the conveying direction of thebelt-like substrate was observed. As to only the back reflector film,interference color due to the nonuniform thickness of the ZnO film wasnot observed.

A similar experiment was performed but the rocking period T of themagnet assembly was shifted to various values from the ideal valuesatisfying p/v=(n+1/m₂)T (wherein m₂=2 and n=4). Very little variationin the short-circuit current and interference colors were observed whenn was within the range of n=4±{fraction (1/16)}. However, if n was outof the above range, large variation up to 3% in the short-circuitcurrent were observed and periodic interference patterns caused bynonuniformity of film thickness were observed.

EXAMPLE 9

A thin Ag film and a thin ZnO film were deposited in the same manner asin Example 7, and a plurality of semiconductor films were successivelydeposited thereon by using the apparatus shown in FIG. 7, therebyproducing photovoltaic devices.

The obtained photovoltaic devices were evaluated. Very little variationin the short-circuit current (Jsc) in the conveying direction of thebelt-like substrate was observed. As to only the back reflector film,interference color due to nonuniform thickness of the ZnO film was notobserved.

A similar experiment was performed but the rocking period T of themagnet assemblies was shifted to various values from the ideal valuesatisfying p/v=(n+1/m₂)T (wherein m₂=2 and n=4). Very little variationin the short-circuit current and interference colors were observed whenn was within the range of n=4±{fraction (1/16)}. However, when n was outof the above range, large variation up to 4% in the short-circuitcurrent were observed and periodic interference patterns caused bynonuniformity of film thickness were observed.

As can be understood from the above description, the present inventionhas the advantage that when a functional deposition film is formed usinga roll-to-roll apparatus, a film with a uniform thickness can becontinuously deposited with a high efficiency in using a target.

What is claimed is:
 1. A method of producing a photovoltaic device,comprising the steps of forming a metal layer and/or a transparent andelectroconductive layer by using a sputtering method comprising thesteps of: forming a plurality of tunnel-shaped magnetic fluxes on atarget; forming an electric field between the target and a belt-shapedsubstrate; and conveying the belt-shaped substrate while reciprocatingthe plurality of tunnel-shaped magnetic fluxes at least in the directionof conveying the belt-shaped substrate, wherein the speed v of conveyingthe substrate, the distance L in the direction of conveying thebelt-shaped substrate between two adjacent points where the magneticfield of the plurality of tunnel-shaped magnetic fluxes and the electricfield cross each other at a right angle, and the period T of areciprocating motion of the plurality of tunnel-shaped magnetic fluxesare controlled so as to satisfy L/v=(n+½)T wherein n is z−{fraction(1/16)}<n<z+{fraction (1/16)} and z is an integer equal to or greaterthan
 0. 2. A method of producing a photovoltaic device, comprising thesteps of: forming a metal layer and/or a transparent andelectroconductive layer by using a sputtering method comprising thesteps of: forming a plurality of tunnel-shaped magnetic fluxes of aclosed loop shape on a target; forming an electric field between thetarget and a belt-shaped substrate; and conveying the belt-shapedsubstrate while reciprocating the plurality of tunnel-shaped magneticfluxes at least in the direction of conveying the belt-shaped substrate,wherein the speed v of conveying the substrate, the interval p ofdisposing the plurality of closed loops, and the period T of thereciprocating motion of the plurality of tunnel-shaped magnetic fluxesare controlled so as to satisfy p/v=(n+1/m₁)T wherein n is z−1/(8m₁)<n<z+1/(8 m₁), z is an integer equal to or greater than 0, and m₁ isa number of closed loops mutually canceling nonuniformity; and forming asemiconductor layer.
 3. A method of producing a photovoltaic device,comprising the steps of: forming a metal layer and/or a transparent andelectroconductive layer by using a sputtering method comprising thesteps of: forming a tunnel-shaped magnetic flux on a target; forming anelectric field between the target and a belt-shaped substrate; andconveying the belt-shaped substrate while reciprocating thetunnel-shaped magnetic flux at least in the direction of conveying thebelt-shaped substrate, wherein the target is present in plurality andthe plurality of targets reciprocate independent of one another, andwherein the speed v of conveying the substrate, the distance d betweentwo adjacent centers of the plurality of independently reciprocatingtargets, and the period T of the reciprocating motion of thetunnel-shaped magnetic flux are controlled so as to satisfyd/v=(n+1/m₂)T wherein n is z−1/(8 m₂)<n<z+1/(8 m₂), z is an integerequal to or greater than 0, and m₂ is a number of targets mutuallycanceling nonuniformity; and forming a semiconductor layer.